If you upgrade your portal from SharePoint Point 2003 to SharePoint Server 2010.you will face many issues , especially if you are using Listing Summary web parts in SharePoint Portal 2003, because when you upgrade from SharePoint 2003 to Microsoft SharePoint Server 2007 , you find that the listing summery web part is obsolete now in MOSS 2007 and it’s replaced by Content Query Webpart

This following article shows the listingsummary class as obsolete in 2007.

We have faced an issue while upgrading SharePoint Portal 2003 to SharePoint Server 2010 .

In this case we have 2 pahse of upgrade , 1st phase is upgrading from SharePoint Portal 2003 to Microsoft Office SharePoint Server 2007, then 2nd phase in upgrading from Microsoft Office SharePoint Server 2007 to SharePoint Server 2010 .

As we know, there were a huge differences between Microsoft Office SharePoint Server 2007 version and SharePoint Portal 2003 version in Architecture Information level such as master pages , page layouts , Shared Service Provider ,.. plus there many features added to MOSS 2007 Version such as InfoPath Forms, BCD,..

So when we upgrade from SharePoint Portal 2003 to MOSS 2007 , many classes and web parts were replaced with a new classes in MOSS 2007 such as HomePage this class was used in SharePoint Portal 2003 and it’s deprecated in MOSS 2007 ( a reference to this class in home page was the reason in Loading Home Page Exception after upgrading to SharePoint Server 2010)

In our case , we upgrade 1st Phase successfully without any issue except some adjustment in home page UI , the portal after is working fine and all sites and inner page also.

Moving to SharePoint Server 2010 was a problem , because as I mentioned above many classes come from SharePoint Portal 2003 is deprecated in Moss 2007 and there are no references to them in SharePoint 2010 Dlls. As we doing an upgrade , the references to these classes are still exist in content DB, so an exception has occurred in loading home Page and inner pages.

This issue was ‘Microsoft.SharePoint.Portal.WebControls.HomePage’ is obsolete, and it is no longer existing in the ‘Microsoft.SharePoint.Portal, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c’, however, it is existing in ‘Microsoft.SharePoint.Portal, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c’.
In this case, it seems you are using the ‘Microsoft.SharePoint.Portal.WebControls.HomePage’ control in the SharePoint 2007(I mean Microsoft Office SharePoint Server(MOSS) 2007). After upgrading, the SharePoint 2010 tries to load the ‘Microsoft.SharePoint.Portal.WebControls.HomePage’ from the assembly with version 14.0.0.0. However, it is not existing. That is what the error means.

So to fix the issue, you need to remove the ‘Microsoft.SharePoint.Portal.WebControls.HomePage’ from SharePoint 2007, and then upgrade the site again.